Referring now to FIG. 4 , a conventional vehicle brake master cylinder, which is disclosed in German laid-open patent publication DE 30 21 893-A published on Dec. 18, 1980, is explained. The brake master cylinder has a reservoir tank (1) and a cylinder (2). The reservoir tank (1) is integrally provided with the cylinder (2). In the cylinder (2), a first piston (3) and a second piston (5) are coaxially disposed. Both the first piston (3) and the second piston (5) are capable of sliding in the cylinder (2) in response to a depression force from a brake pedal (not shown). The first piston (3) receives the depressing force from the brake pedal via a brake booster (not shown). The second piston (5) receives the axial pressure from the first piston (3) via a first return spring (4). Further, the second piston (5) supports a second return spring (7). The second return spring (7) is inserted in a hollow portion (6) of the second piston (5). The second return spring (7) is pinched between a bottom (8) of the cylinder (2) and the second piston (5). A projection (8a) is formed on the bottom (8) of the cylinder (2). The projection (8a) sustains the spring (7) coaxially with respect to the cylinder (2). A length of the cylinder (2) can be reduced by inserting one end of the spring (7) into the hollow portion (6) of the second piston (5).
A first pressure chamber (9) is formed between the first piston (3) and the second piston (5). A second pressure chamber (10) is formed between the the bottom (8) of the cylinder (2) and the second piston (5). The first pressure chamber (9) and the second pressure chamber (10) are connected to the reservoir tank (1) via communication ports (11) and (12). The first pressure chamber (9) and the second pressure chamber (10) are connected to the brake pipes (not shown) via outlet ports (not shown) and supply brake pressure to each linkage of the vehicle brake system. Further, a first supply chamber (13) is formed around the first piston (3). A second supply chamber (14) is formed around the second piston (5). The first supply chamber (13) and the second supply chamber (14) are connected to the reservoir tank (1). The first supply chamber (13) is capable of communicating with the first pressure chamber (9) when the first pressure chamber (9) has a lower pressure than the first supply chamber (13). The second supply chamber (14) is capable of communicating with the second pressure chamber (10) when the second pressure chamber (10) has a lower pressure than the second supply chamber (14).
However, when vibration is affected to the master cylinder from the vehicle body, the second return spring (7) moves rapidly to and fro. A middle part of the second return spring (7), which almost locates an open end of the second piston (5), moves most intensely. Due to the rapid movements of the second return spring (7), the hollow portion (6) of the second piston (5) may be scratched by the second return spring (7). Further, unpleasant noise may be generated due to interference between the second return spring (7) and the second piston (5). When the second return spring (7) scratches the second piston (5), very small particles are removed from both the second return spring (7) and the second piston (5). These small particles may slightly deteriorate the ability of the primary cups (15) and (16) to remain fluid tight.
Further, the conventional master cylinder is hard to assemble because the second return spring (7) and the second piston (5) must be located very carefully in the narrow cylinder (2). Therefore, a long period of time is required to assemble the master cylinder (2) and thus the master cylinder (2) becomes expensive.